RadioBanter - View Single Post

March 9th 04, 06:12 AM

In article ,
(Mark Keith) wrote:

Telamon wrote in message
.
..
In article ,
"DJB" wrote:

Question to the group,

Is it better to have a separate ground for you random wire short
wave antenna (w/9:1 transformer) and a separate ground for your
receiver (station ground) or can both use the same ground? If
both were connected to the same ground, wouldn't that cause
ground loop problems?

Thank you in advance

It prevents a ground loop problem to use isolated grounds.

Huh???? This doesn't make sense to me. Using separate grounds is the
fastest way to cause a ground loop. All grounds should always be tied
together at a common point. It's in the NEC. And all gear should use
it's own single wire to that single ground point. "star
configuration" This assures no ground loops.

It will help reduce noise from the AC powering your radio from
being part of the ³measurement³ in this case the signal of the
station you are trying to receive at the input to the radio from
also seeing noise on the AC power system.

How will grounding a radio reduce AC power noise? If you have AC
noise, you need AC line filtering, not a ground. Or seems to me
anyway...I haven't grounded any of my radios in years. I don't have
AC noise problems, unless I pick up radiated line noise. No grounding
will cure that.

Creating a ground
for the antenna that is isolated from the power ground through the
³transformer² will help reduce the noise floor on received signals.

I don't see how. But even ignoring that, what you propose is a
violation of NEC. And it's also a way to ensure severe lightning
damage if that ever occurs due to the differences in ground
potentials.

If you built a balanced or complete antenna like a dipole the
antenna ground would not help much at all.

True.

The transformer would still help to
isolate the antenna circuit from the virtual RF ground to the AC
system reducing the noise floor.

This confuses me though...What virtual RF ground? Normally, the AC
system should not be an issue if using a dipole. Best way to ensure a
quiet dipole is to use good decoupling. IE: balun, chokes, etc to
reduce noise ingress... Myself, and this is open to debate of
course...I would use a single ground point. I would use a ground rod
at the balun. And that ground would be bonded to the main house
ground system to keep at the same potential. I probably would not
bother grounding the radio, but if I did, I would use a single wire
to the same ground outside the shack at the balun. There is no way
this will cause a ground loop. And will reduce problems if lightning
strikes. Remember the recent lightning post where all gear in the
house was trashed? Thats what happens when you don't bond all grounds
together to keep them at the same potential. Of course, I don't
consider mine the last word, but I do have to respectably disagree
with your separate grounding proposal...I would never do that myself. MK

Besides multiple questions through this post you already posted to the
thread two other times so I¹ll try to clarify at the end of this one.

There are several problems that need to be solved with a Marconi type
antenna the main one being to not use the mains ground as the antenna
ground. Using the mains ground means noise in the power system will be
part of the signal input. A Marconi type needs a ground to be complete
so you make one for the single element antenna. The basic idea being
ground rod with or without radials together with the random wire is the
antenna system. You make a connection to the coax through a transformer
one side has the antenna ground for a return path and the other a power
mains path from the radio. The transformer isolates the grounds. The
type of UNUN transformer here would be a voltage type for very long
wires and I would use a choke type for shorter random wires.

If you have the power mains ground connected to the antenna ground you
will have a ground loop. The concept is that a ground of the
measurement device (the radio) influences the signal. The problem
become worse as you add measurement devices with their own grounds.
Normally in a test setup all grounds would be brought together at the
DUT in a unified ground but that can¹t be done here.

The mains radio ground could be a direct DC connection or through a
capacitor which would be virtual or the radio may have a separate
terminal for antenna ground using an input transformer of various types
depending on the design.

The forgoing only considers the RF nature of what needs to be
accomplished and has no consideration for lightning protection, which
is a different issue altogether.

I¹m no expert on the lightning issue since it is pretty rare in
southern California but if I were in lightning country I would move.

Just kidding.

For lightning protection you would need an additional ground at the
side of the house where the antenna coax comes in besides the antenna
ground. At that ground the coax shield would be connected and a shunt
protection device like a gas tube would connect the center conductor to
the same ground. For the lightning ground to be complete you would have
to bring the AC mains power to the radio to this same ground. The mains
ground is also connected to this ground. The supply and return AC mains
wires would have shunt protection devices to this same ground and
between them. Now all wires to the radio will be referenced to the same
ground at the side of the house and lightning induced voltage will be
equalized at that ground on all wires going to the radio. I don¹t know
the electrical code in this regard and this side of the house lightning
ground may have to be the power entrance to the house to be legal. If a
big storm is coming I think most people would be better off
disconnecting the antenna and power to the radio.

A dipole antenna is a complete antenna and does not need a ground for
the RF signal aspect. I would favor a current BALUN for any complete
antenna type, which would provide isolation from the radio mains ground
and impedance transformation if needed.

--
Telamon
Ventura, California